Select Max() with "group by" in mongodb

mongodb

Solution

I have created Mongo Collection as follows.

{ "_id" : ObjectId("4fb36bfd3d1c88bfa15103b1"), "name" : "bob", "value" : 5 }
{ "_id" : ObjectId("4fb36c033d1c88bfa15103b2"), "name" : "bob", "value" : 3 }
{ "_id" : ObjectId("4fb36c063d1c88bfa15103b3"), "name" : "bob", "value" : 7 }
{ "_id" : ObjectId("4fb36c0c3d1c88bfa15103b4"), "name" : "john", "value" : 2 }
{ "_id" : ObjectId("4fb36c103d1c88bfa15103b5"), "name" : "john", "value" : 4 }
{ "_id" : ObjectId("4fb36c143d1c88bfa15103b6"), "name" : "john", "value" : 8 }
{ "_id" : ObjectId("4fb36c163d1c88bfa15103b7"), "name" : "john", "value" : 6 }

Then by using the following code I group it by their name and max(value)

db.table1.group(
    {key: {name:true},
        reduce: function(obj,prev) { 
            if (prev.maxValue < obj.value) { 
                prev.maxValue = obj.value; 
            }  
        },
    initial: { maxValue: 0 }}
);

The result is shown as

[
    {
        "name" : "bob",
        "maxValue" : 7
    },
    {
        "name" : "john",
        "maxValue" : 8
    }
]

It is much simpler with the aggregation framework. You can get the same result with the following code by using aggregation framework.

db.table1.aggregate([
    {
       $group:{_id:"$name", "maxValue": {$max:"$value"}}
    }
]);

Problem

Please help me to convert this select sentence to mongodb: ``` Select Name, Max(Value) From table1 Group By Name ``` I read this document: http://www.mongodb.org/display/DOCS/Aggregation#Aggregation-Group but still dont know how to apply Max() method instead SUM() as that document. Thank you.

Original source